Field of the Invention
The invention relates to a fiber preform for a turbine sector, to its method of fabrication, to a turbine ring sector incorporating such a fiber preform, and to a turbine ring assembly comprising a plurality of such ring sectors.
The field of application of the invention is particularly that of gas turbine aeroengines. Nevertheless, the invention is applicable to other types of turbomachinery, e.g. industrial turbines.
Description of the Related Art
Ceramic matrix composite (CMC) materials are known for their good mechanical properties that make them suitable for constituting structural elements, and for their ability to conserve their properties at high temperatures.
In gas turbine aeroengines, improving efficiency and reducing polluting emissions has lead to a search for operation at ever-higher temperatures.
Thus, the use of CMCs for the various hot parts of such engines has already been envisaged, particularly since CMCs are of lower density than the metallic materials conventionally used.
Thus, turbine ring sectors have already been made as a single piece of CMC with a structure of the kind shown in FIG. 1.
FIG. 1 shows a high pressure turbine ring assembly comprising a CMC turbine ring 10 and a metal ring-support structure 13. The turbine ring 10 surrounds a set of rotary blades 15. The turbine ring 10 is made up of a plurality of ring sectors 100, FIG. 1 being a meridian section view on a plane passing between two contiguous ring sectors.
Each ring sector 100 has a section that is substantially in the shape of an upside-down Greek letter pi (π) with a base 112 in the form of a ring segment and an inside face coated in a layer 113 of abradable material defining the flow passage for the gas stream in the turbine. Tabs 114 and 116 extend from the outside face of the base 112. One of the tabs, the upstream tab 114, presents a meridian section that is substantially in the form of a letter L on its side and it extends upstream so as to co-operate with the upstream end of the base 112 to define a U-shaped portion defining a housing 115 extending over the depth of the upstream end portion 114a. The other tab 116, or downstream tab, presents a meridian section that is substantially in the shape of a letter T on one side and it extends downstream from the downstream end of the base 112.
The terms “upstream” and “downstream” are used herein with reference to the flow direction of the gas stream through the turbine (arrow F in FIG. 1).
The ring support structure 13, which is secured to the turbine casing 30, comprises an annular upstream radial flange 32 carrying a hook 34 of annular shape with a comma-shaped meridian section pointing downstream in the axial direction, and suitable for engaging in the housing 115. The way the end portion 34a of the hook 34 engages in the housing 115 is designed so as to provide sealing between the gas stream flow passage and the outside of the ring sectors 100, on their upstream end.
On the downstream end, the ring support structure 13 includes an annular flange 36 of upside-down L-shape terminated by an annular bearing portion 36a against which the annular end portion 116a of the tab 116 bears. The end portion 116a of the tab 116 and the bearing portion 36a of the flange 36 are held pressed against each other without clearance by means of a clamp 38 having a meridian section in the shape of a letter U on its side, forming a spring clip in conventional manner. The clamp 38 is held stationary in circumferential manner relative to the flange 36 and the tab 116 by being inserted between fingers 36b and 116b that project downstream from the portion 36a of the flange 36 and from the end portion 116a of the tab 116.
In order to provide the best possible sealing between the gas flow passage through the turbine and the outside of the turbine at its downstream end, a gasket 20 is compressed between the bearing portion 36a of the flange 36 and the end portion 116a of the tab 116. By way of example, the gasket 20 is constituted by a metal braid held in a housing formed in the inside face of the bearing portion 36a. 
In addition, inter-sector sealing is provided by sealing tongues 22 housed in each ring sector 100, inside the base 112, in its middle portion and over almost the entire upstream to downstream length of the base 112.
Ring sectors 100 as described above need to present significant thickness so that the portions made of CMC surrounding the sealing tongue and also constituting the two tabs 114 and 116 are strong enough to withstand the thermomechanical stresses to which they are subjected in operation.